Injection molding apparatus

ABSTRACT

An integrated circuit encapsulation apparatus comprises a molding press, which has a mold defining a cavity adapted to receive an integrated circuit die and an attached leadframe for encapsulation thereof. The molding press is operable by an electro-pneumatic driving mechanism which utilises a geared servo motor for opening and closing the mold, and a pneumatic cylinder for providing clamping pressure. Both the servo motor and pneumatic cylinder act upon a threaded screw member for movement of the molding press. The press is provided with an integrated mold brushing unit which has transversely rotating brushes and travels over the mold faces to remove debris. In a further refinement, a spring-loaded bearing system is provided for easy removal of the mold.

This invention relates to apparatus for injection molding, for example,for encapsulating integrated circuits.

BACKGROUND OF THE INVENTION

In the electronics manufacturing industry, a high degree of cleanlinessis required to prevent contamination of circuitry and devices, which maycause subsequent electronic failure. A common way of packagingelectronic integrated circuits is by encapsulating the prefabricatedintegrated circuit and a portion of the attached leadframe in a plasticsmaterial. Typically this is performed utilising injection moldingapparatus, often of the transfer mold variety. Known injection moldingequipment employs hydraulic presses, and it is not uncommon forhydraulic fluids to leak from the hydraulic presses, which maycontaminate the molds and molded products. Although cleansing of themolds is performed regularly, the cleaning techniques are not alwayseffective in removing contaminating debris from the molds. Furthermore,removal of molds for cleaning is often difficult and time consuming. Itis an object of the present invention to overcome these problems.

BRIEF SUMMARY OF THE INVENTION

Accordingly the present invention provides an injection moldingapparatus, comprising a molding press adapted to receive a mold having acavity shaped for the formation of a molding product, the moldingapparatus comprising: first and second platens which are movablerelative to one another so as to allow respective mold parts mountedthereon to be moved between open and closed configurations; and

an electro-pneumatic drive mechanism comprising: a threaded screw membercoupled to one of the first and second platens; an electric motorcoupled to drive the threaded screw member by way of a gear mechanismfor moving the first and second platens relative to one another; and apneumatic cylinder mechanism for driving the screw thread of thethreaded member separately from said electric motor.

The present invention is particularly well adapted for use in anintegrated circuit encapsulation process.

Preferably, the mold is provided with a spring-loaded bearing system foreasy removal of the mold from the press.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail below by way of exampleonly, with reference to the accompanying drawings in which:

FIGS. 1A, 1B and 1C are respective cross-sectional views through anexemplary molding press illustrating the operation for encapsulating anintegrated circuit and leadframe;

FIGS. 2A and 2B show a plan and cross-sectional elevation viewrespectively of an electro pneumatic press;

FIGS. 3A and 3B show an elevation and side cross-sectional elevationrespectively of a mold brushing system; and

FIGS. 4A and 4B are cross-sectional views showing a spring-loadedbearing system for removing the mold.

FIGS. 1A, 1B and 1C are cross-sectional views of an exemplary transfermolding press 1 adapted to receive two molds 2. Each mold 2 is arrangedwithin the molding press 1, and comprises upper and lower mold parts 2a, 2 b which fit together to define a mold cavity 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The molding press 1 is shown in FIG. 1A in a closed position, havingbeen loaded with integrated circuit leadframes 6 within the respectivemold cavities 4, and a pellet of encapsulating material 3 in a gangpot9. Encapsulation of the integrated circuits 6 is achieved by heating theencapsulating material pellet 3 and pressing it within the gangpot usinga transfer plunger 8, which causes the pellet 3 to liquefy and flow intothe mold cavities 4 through small passages between the gangpot and themold cavities (see FIG. 1B). After allowing the encapsulating materialto solidify again, the molding press 1 is opened (FIG. 1C), wherein themold parts 2 a, 2 b are separated. The encapsulated integrated circuits7 are lifted from the mold cavity by way of ejector pins 10, so as toexpose them for removal from the molding press. After removal of theencapsulated integrated circuits 7, the open molding press is ready toreceive new leadframe inserts 6 and encapsulating material pellet 3 torepeat the encapsulating process.

In order to move the mold parts 2 a, 2 b towards and away from eachother between the open and closed configurations illustrated in FIGS. 1Cand 1A, respectively, a driving mechanism is required. The upper andlower mold parts 2 a, 2 b are respectively mounted on upper and lowerplatens 16 a, 16 b of the molding press, and the upper and lower platensare movable relative to one another along linear guide rods 20 (see FIG.2B). Typically hydraulic driving mechanisms have been employed formovement of the molding press platens, and in the electronicsencapsulation industry, at least, such driving mechanisms are subject todisadvantages as discussed hereinabove. However, hydraulic drivingmechanisms have persisted even in the electronics encapsulation industrybecause other characteristics of the hydraulic systems make them wellsuited to the requirements for driving molding presses. In particular,hydraulic systems are able to easily provide a sufficient range ofrelative movement of the platens to permit access to the mold cavitieswhen separated, and also enable generation of a sufficiently largeclamping force between the mold parts when the press is closed.

The molding press driving mechanism of the present invention is able toprovide the range of movement and force requirements of the moldingpress, but without the deleterious effects which can result from theprior art hydraulic driving systems. A particular preferred form of themolding press driving mechanism is illustrated in FIGS. 2A and 2B, inplan and cross-sectional elevation views, respectively. Representationsof the upper and lower mold press platens are shown at 16 a and 16 b(FIG. 2B), mounted for relative movement along guide rods 20. In thisdriving mechanism, it is the upper platen 16 a which is in fact movablealong the guide rods 20, to effect displacement relative to the lowerplaten 16 b which is fixed to the guide rods. The driving mechanismcomprises an electro-pneumatic system, since the two operativecomponents are an electrically activated servo motor and a pneumaticallymotivated cylinder and piston arrangement.

The upper platen 16 a has an elongate ball-screw 15 mounted thereon,centrally arranged on top of the platen and rotatable about a centralaxis. The ball-screw 15 extends parallel to the guide rods 20 away fromthe lower platen 16 b, and has an external screw thread formed thereon.A fixed frame plate 40 is mounted at the top of the guide rods 20 havinga central aperture aligned with the ball-screw 15. Within the centralaperture is mounted a flange member 42 which is fixed to preventmovement thereof relative to the frame plate 40 in the axial directionof the ballscrew, but to allow rotational movement about the ball-screwaxis. The flange member 42 has a circular internally threaded opening,and the ball-screw 15 extends through the opening with the internal andexternal screw threads of the ball-screw and flange member interfitting.Accordingly, relative rotational movement between the ball-screw 15 andflange member 42 is translated into axial movement of the ball-screwrelative to the frame plate, the direction of rotation determining thedirection of axial movement. In view of the construction, therefore,relative rotational movement between the ballscrew 15 and the flangemember results in relative linear movement of the upper platen 16 a,with respect to the lower platen 16 b, along the guide rods 20.

As mentioned, the electro-pneumatic driving mechanism provides twoactive components, namely an electric servo motor 14, and a pneumaticpiston and cylinder arrangement 17. Both the electric and pneumaticactive components operate upon the ball screw arrangement abovedescribed, but provide different functions. In particular, the servomotor 14 is arranged to provide the required range of relative movementof the platens to permit sufficient access to the mould cavities whenseparated, whilst the pneumatic piston and cylinder is arranged toprovide a sufficiently large clamping force between the mold parts whenthe press is closed.

Adjacent the top of the upper platen 16 a, a gear cog 12 is mounted foraxial movement with the ball-screw 15. The gear cog 12 intermeshes witha driving cog 11 which is driven by the servo motor 14 mounted on theupper platen 16 a. Accordingly, driving the servo motor 14 rotates thedriving cog 11 which thereby causes rotational movement of the gear cog12 and ball screw 15. As discussed above, this rotational movementresults in linear axial movement of the ball-screw 15 and upper platen16 a, relative to the lower platen 16 b. Thus, by controllably drivingthe servo motor 14, the upper platen 16 a can be moved relative to thelower platen 16 b to, in use, open and close the molds of the moldingpress. Whilst the ball-screw 15 is driven by the servo motor 14 theflange member 42 of course remains stationary.

When the molds of the molding press are positioned in the closedconfiguration by action of the servo motor 14, it is then desired toensure that sufficient clamping force is applied between the upper andlower mold parts. This function is provided by the pneumatic cylinderand piston arrangement 17. A lever arm 18 is fitted to the flange member42 by way of connecting bolts 19, and a movable end of the piston of thecylinder and piston arrangement 17 is coupled to the end of the leverarm 18. This arrangement enables movement of the piston by action ofincreased pneumatic pressure in the cylinder 17 to be translated torotational movement of the lever arm 18, and thus to the flange member42. An interlocking device 13 is also provided adjacent the gear cog 12,which is operable to interlock with the gear cog 12 to prevent rotationthereof as well as the ball screw 15. Thus, motion of the piston 17which results in rotational movement of the lever arm 18 and flangemember is translated to axial movement of the ball screw 15. However,because of the nature of the mechanism the pneumatic cylinder is onlyable to produce a small rotational movement of the flange member 42,which results in only a small axial movement of the upper platen,although significant clamping force can nevertheless be generated.

Utilising the construction of the driving mechanism illustrated in FIGS.2A and 2B, the servo motor 14 is able to be controlled so as to move theupper and lower latens 16 a, 16 b apart and together between open andclosed configurations of the molds. When the servo motor is controlledso as to configure the molds into a closed configuration, then thepneumatic cylinder 17 can be activated by increasing the gas pressuretherein so as to rotate the flange member relative to the ball screw 15.This action, combined with activation of the interlocking device 13 toprevent rotation of the ball screw 15 enables a clamping pressure to beapplied by the pneumatic cylinder 17.

In FIG. 3A, heat resistant brushes 21 are mounted along a series ofparallel endless V-belts 23 which run on pulleys 22. A motor 28 drivesthe pulleys 22 through a drive mechanism 26. When the mold is opened,the brushing unit rotate in one direction is movable axially of thepulleys 22 (ie. transverse to the movement of brushes 21) across themold-face brushing any debris away from the mold and then rotate in theanother direction during the returning stroke of the cleaning process.The rotation of the brushes about a horizontal axis as the translationoccurs provides a more effective cleaning action than that of knowncleaning methods which have brushes rotating about vertical axes.

In FIGS. 4A and 4B, a roller-bearing system 34 is mounted in thepress-table 36 and is loaded with a spring 33 set in the base plate 32.When the mold 31 is tightened, it depresses the spring 33 and sits incontact with the press table 36. When the mold is released, the spring33 uncoils and raises the mold above the press table 36 allowing easyremoval. The temperature of the mold which can be as high as 180° C.makes mold changing a difficult operation and this system increases thespeed of changing considerably. As shown in FIG. 4A, the strength ofsprings 33 is such as to elevate the mold 31 by pressure of the bearings34 to about 1 mm above the surface of the press table 36. It will beimmediately apparent that the bearings 34 may be of a form comprisingrollers, or incorporating a ball bearing to allow two dimensionalrelative movement between the press table and mold, as will beappreciated by those skilled in the art. It is of course obvious thatthe roller-bearing system would be incorporated in the lower platen 16b, and that the base plate 32 and press table 36 may comprise componentsthereof.

The above detailed description is by way of example only, and is notintended to limit the scope of the invention which is defined in thefollowing claims.

The claims defining the invention are as follows:
 1. An injectionmolding apparatus comprising: a frame; a first platen and a secondplaten, the first platen and the second platen adapted to receive a moldhaving mold parts that form a cavity shaped for the formation of amolding product wherein the first platen is movable relative to thesecond platen and said frame, so as to allow the mold parts mounted onsaid first and second platens to be moved between an open position and aclosed position; a first drive mechanism including an electric motor anda gear mechanism coupled to the electric motor and mounted on said firstplaten; a threaded screw coupled to the first platen, the threaded screwbeing engaged with the gear mechanism whereby a rotary force generatedby the motor drives the threaded screw for moving the first platenrelative to said second platen and said frame; a flange mounted on saidframe and threadedly engaged with the threaded screw; a pneumaticcylinder and a lever mechanism mounted on said frame and arranged toapply torque through the lever mechanism to the flange threadedlyengaged with the threaded screw to provide a clamping force between themold parts when said mold is in the closed position; and wherein thepneumatic cylinder and the lever mechanism drive a portion of thethreaded screw independently of the electric motor.
 2. The injectionmolding apparatus as claimed in claim 1, wherein the threaded screw hasa length and an axis with an axial direction along said length, saidframe has a frame plate and the flange has a circular internallythreaded opening for threadedly receiving the threaded screw, the flangebeing fixed to the frame plate to prevent movement thereof relative tothe frame plate in an axial direction of the threaded screw, but toallow rotational movement about said axis of the threaded screw.
 3. Theinjection molding apparatus as claimed in claim 1, wherein the pneumaticcylinder includes a piston rod, and wherein the lever mechanism isfixedly connected to the flange and is adapted to be activated by thepiston rod of the pneumatic cylinder.
 4. A molding apparatus,comprising: a frame; a first platen and second platen, the first and thesecond platens being adapted to receive a mold having mold parts thatform a cavity shaped for the formation of a molded product, wherein thefirst platen is movable relative to the second platen and said frame, soas to allow the mold parts mounted on the first and second platens to bemoved between an open position and a closed position; a first drivemechanism; a threaded screw coupled to the first platen and to saidfirst drive mechanism, whereby a rotary force generated by said firstdrive mechanism drives the threaded screw to move said first platenrelative to said second platen and said frame; and a second drivemechanism including a lever and arranged to apply torque through saidlever to the threaded screw to provide a clamping force between the moldparts when said mold parts are in the closed position; wherein the firstdrive mechanism and the second drive mechanism are operable to drive thethreaded screw independently; and wherein one of said first drivemechanism and said second drive mechanism is mounted on said frame andthe other of said first drive mechanism and said second drive mechanismis mounted on said first platen.
 5. The molding apparatus as claimed inclaim 4, wherein said threaded screw has length and an axis with anaxial direction along said length; said frame has a frame plate and nutwith a circular internally threaded opening threadedly receiving thethreaded screw; said nut is mounted in said frame plate to preventmovement thereof relative to the frame plate in an axial direction ofthe threaded screw, but to allow rotational movement about the axis ofsaid threaded screw; and the nut is rotated by the one of said firstdrive mechanism and said second drive mechanism mounted on said frame todrive said threaded screw in the axial direction of the threaded screw.6. The molding apparatus as claimed in claim 5, wherein the second drivemechanism further includes a cylinder including a piston rod, andwherein the lever is fixedly connected to the nut and is adapted to beactivated by said piston rod of the cylinder.